It's a long time that I've searched for a night light for T5 4W tubes, but all those that I've found used capacitive ballast (=lamp killer) and not electronic one (magnetic is not really an option on 230V country, the losses on a 4W lamp with ~30V arc voltage are way too much). So I've decided to build one.
I've used a plastic profile found in a DO-it shop as fitting body, a standard Swiss plug with the earth removed, a generic switch, Vossloh Schwabe lamp holders and ballast.
The ballast is based on the standard self oscillating circuit, but with added the eol protection and a PTC resistor for preheating.
The first mod that I've made to the ballast is to add a sidac diode in series with the PTC resistor to avoid the constant heating (= useless power consumption) of the PTC when the lamp is running (for details see my other post on the subject).
The second mod was to decrease slightly the current supplied to the lamp: as the ballast is rated for 4-6-8W lamps it overdrive a little the 4W tubes. The correct lamp power on HF ballast is 3.6W with an arc voltage of 24V (data from Osram catalog), the VS ballast feeds the lamp with 4W(value from datasheet) with an arc voltage of 22.2V(value measured).
To decrease the lamp current I've added 30 turns of wire on the original inductor (I was lucky because there was space for more turns), the total power consumption has decreased from 5.2W to 4.5W and the lamp voltage has increased from 22.2V to 23.5V.
Now I'm happy with my new night light, I use it every night to light the bathroom and works very well. At the moment I use an well used old italian made Osram warm white tube.